Burner elements have long been used to heat fluids in commercial, industrial and residential applications. In the past, flat plenum clay tiles were used to pass gas mixtures for burning on the surface of the element. These heaters are undesirable because they lose heat to the environment and they heat undesirable sections of the system. Cylindrical heaters have been found to be more efficient, but they require a fine balance of pressurized gases and uniformity so that the flame does not creep back into the mixing chamber and explode or backflash.
In selecting a material to be used for distributing gas in a burner element, the following criteria have been found to be important:
1. The material should allow for a low pressure drop. PA1 2. The material must have uniform openings for evenly distributing the gas mixture. PA1 3. The material should have good insulative properties to prevent backflashing. If heat from the flame creeps back into the inner portion of the element, the gas ignites inside the element and backflashes. PA1 4. When using propane gas, a material is needed which ignites strictly on the outer surface. All gases should ignite on the surface, however since propane has a higher flame velocity than natural gas (2.85 ft/sec to 1 ft/sec) its flame has a higher tendency to creep into the pore. In other words, it burns closer to the surface than a natural gas flame.
The general problems experienced by prior art devices included non-uniform pore sizes, problems with high back pressures and backflashing when flames crept back to the source of the combustion gas. Recently, ceramic fibers have been used to make the cylindrical burner elements, but these fibrous elements lack integrity. Ceramic fiber burner elements are made by placing wet ceramic fiber over a predetermined size mesh, and vacuuming the moisture out to form a cylinder of fibers. Because the fiber matrix must be thin enough to allow gas to pass therethrough, the strength of the matrix is compromised and the material subsequently degrades. Furthermore, the filter-like structures clog with dust and impurities carried by the gas, and the filters require higher pressures to force the gas therethrough.
In the very recent past, ceramic foam elements have been used for constructing burners. Ceramic foam is made by soaking a polyurethane foam or other combustible foam material with a liquid ceramic material, drying it and burning off the foam material to leave a porous ceramic structure. By selecting the proper pore size of the precursor foam, the number of pores per linear inch may be predetermined for desired results. These ceramic foam materials were developed for filtering high temperature casting alloys. The following patents are illustrative of previous attempts to remedy the above-mentioned patents.
Cooper U.S. Pat. No. 4,608,012 issued Aug. 26, 1986 discloses a gas burner made of cylindrical reticulated ceramic foam with porosities in the range of 15-40 pores per linear inch. In that disclosure, it was stated that ceramic foam materials having a porosity of 45 pores per linear 25 mm (about one inch) would not pass a sufficient quantity of gas/air mixture to provide stable combustion because the pore size was too small and excessive back pressure was created in the mixing chamber, thereby preventing sufficient air from being induced to provide the correct ratios of gas/air mixtures. It was furthermore disclosed that the best results had been obtained with a porosity of about 30 pores per linear 25 mm.
Weiss U.S. Pat. No. 3,191,659 issued June 29, 1965 discloses a catalyst or thermocatalytically active material applied to a fibrous support to form a coherent layer or body of relatively large thermocatalytically active surface dimension transmissive to fuel vapors. In FIG. 5, tube 13 introduces a more or less uniformly diffused stream of air-gas mixture into the element by virtue of the perforations 14.
Craig U.S. Pat. No. 4,400,152 issued Aug. 23, 1983 to Craig, et al., discloses a porous ceramic heating reactor positioned within a tubular casing to contain a flame and the end products of combustion. The size and shape of the tubular casing were designed to provide an explosion proof, leak proof, efficient heat transfer device.
Craig et al. U.S. Pat. No. 4,416,619 issued Nov. 22, 1983 discloses a porous ceramic reactor used in a heating system. The outer surface of the ceramic reactor has a plurality of indentations which breaks up the surface continuity. The indentations were designed to minimize the scaling and cracking that occurs due to differential expansion and contraction. It was stated that if the surface of the reactor could be broken up in a thermal sense into individual areas or parcels, the limited surface area of each parcel would not expand or contract enough to cause a crack. If a crack did develop, its extent would have been limited by the boundary of the individual parcel.
Morris U.S. Pat. No. 4,568,595 issued Feb. 4, 1986 discloses a ceramic structure for filtering hot fluids such as diesel exhaust or liquid metals comprising a reticulated ceramic portion defined by a plurality of interconnecting webs having a pore distribution of between 5-125 pores per linear inch and a ceramic coating portion sintered to the webs along a surface defined by one face of the coating. The coating has a thickness less than about 3 mm with a ratio of average thickness of the coating to the thickness of the web forming the reticulated ceramic foam between 1-10. The coating is sintered to the flame and is matched to minimize thermal shock.
Accordingly, it is the primary aim of the present invention to provide a highly efficient cylindrical radiant heat burner for a fluid immersion apparatus which will uniformly burn combustion gases without backflashing. The present invention is designed to provide enhanced uniform heating, while reducing fuel consumption and reducing manufacturing costs due to the one-piece construction.